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Canadian Journal of Anesthesia, Vol 31, 460-465, Copyright © 1984 by Canadian Anesthesiologists' Society

Technical Communication: Demonstration of a Problem in Estimating Sensible Heat Loss from the Respiratory Tract by Thermometry

F. G. KING MD FRCP(C)1, H. J. MANSON MB CHB FFARCS, FRCP(C)1, J. W. SNELLEN MD PHD1, and K. S. CHANG PHD1

1 Discipline of Anaesthesia and the Calorimetry Laboratory, Basic Sciences Division, Faculty of Medicine, Memorial University of Newfoundland, St. John's

Address correspondence to: Dr. F.G. King, Discipline of Anaesthesia, Faculty of Medicine, Health Sciences Centre, Memorial University of Newfoundland, St. John's, Newfoundland, A1B 3V6.

We have investigated sensible respiratory loss, which is usually taken as the product of expired volume and the temperature difference between inspired and expired air (Ve x {delta}T). Air temperature was measured with a 0.122 mm copper-constantan thermocouple mounted in the mouthpiece of a T-piece breathing system, and expired volume with a pneumotachograph. Changing air temperature ({delta}T) at the mouth and expired air volume (Ve) were recorded simultaneously while the subject voluntarily breathed at different tidal volumes and rates. Inspired temperatures were controlled at 12.05°C, 21.80° C and 25.74° C at a low dewpoint temperature of 4-5° C.

Temperature volume "loops" were constructed using an x-y plotter. The areas of each "loop" and enclosing rectangle (Ve x {delta}T) were measured. The difference was divided by the weight of the rectangle to give the percentage of overestimation of sensible heat loss, which ranged from 5.5 to 17.2 per cent. The error increased significantly with decreasing tidal volume and increasing respiratory rate.

Key Words: MEASUREMENT TECHNIQUES: respiratory heat loss






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Copyright © 1984 by the Canadian Anesthesiologists' Society.